The subject-matter of this invention consists of an intrinsic-safety nuclear reactor of the pressurized water type.
Canadian patent no. 1.070.860 describes a nuclear reactor of the type with pressurized light water, called the intrinsic-safety type. According to said patent, the vessel containing the reactor core, made of steel and externally insulated, is immersed in a pool provided with its own containment shell. The reactor vessel has at the top an output header for the water which has crossed through the core and got heated, and which, by means of a suitable delivery pipe is conveyed outside the pool to a heat exchanger. From the heat exchanger, the water is conveyed back through a suitable return pipe to an input header located below the core, in the reactor vessel. On the primary circuit return pipe, furthermore, there is a circulation pump. The reactor core, the two headers, the output pipe and the return pipe with the relevant circulation pump, and finally the heat exchanger, form the reactor primary circuit.
In the Canadian patent mentioned above, intrinsic safety is ensured by the fact that the water in the pool is pressurized, and there are means of connection which, in emergency conditions, allow the water from the pool to flow freely into the lower header on the one hand, and means of connection which allow the free flow of the water in the upper header towards the pool, on the other. The emergency conditions envisaged could consist, for example, of a failure of the primary circuit circulation pump, with a consequent increase of the temperature inside the reactor.
The means of connection between the water in the pool and the lower header consist of a pneumatic seal or even an open pipe in which a flow rate of nil is ensured, in normal operating conditions, by means of a suitable play of pressures, as explained below. The means of connection between the upper header and the water of the pool consist of a bell of gas or steam under pressure, installed on top of a fairly high chamber, also full of gas or steam: the height of said chamber must be such that the corresponding head of liquid contained in the pool is equal to the pressure drop in the primary liquid circulating in the reactor. In this way the lower header of the reactor and the surrounding water of the pool are the same pressure, and there is no difference in pressure between the two areas: in spite of the fact that these two areas communicate freely, since their pressures are the same, the flow rate of liquid between one and the other is nil.
In case of failure of the circulation pump, the pressure drop between the lower header and the upper header is eliminated; in particular, the pressure in the upper header increases and the water of the reactor is pushed into the chamber full of gas, and from here into the pool. At the same time, the water from the pool enters the lower header and from here passes into the core. The water of the reactor is therefore replaced by the water from the pool, which is colder: it has already been said that the walls of the reactor are insulated. In addition to this, the water in the pool is borated water so that on reaching the reactor core it gradually stops the reaction.
The volume of water present in the pool is relatively large, and this allows quite a number of hours of primary fluid circulation pump failure without the rector core heating over the pre-established safety limits.
From a strictly technical point of view, the operation of the intrinsic-safety reactor described above and claimed in Canadian patent no. 1.070.860 is unexceptionable. This known reactor, however, has the drawback that it entails a complex construction in the event of using a high-temperature reactor. Indeed, the pressure of the liquid contained in the pool must be higher that the pressure corresponding to the saturation temperature of the fluid on leaving the core, and therefore:
either the quantity of water in the pool is limited, and in this case shut-down of the reactor is ensured but cooling of the core is ensures only in the short term, PA1 or the quantity of water in the pool is large, and in this case complex reinforced concrete structures are needed to guarantee containment of said fluid under pressure.